Chemical reduction of the highly active quinone-containing antitumor drugs, adriamycin and daunorubicin formed the same partially reduced free radical previously reported by microsomal activation. In vitro incubation of the chemically activated free radical intermediates with DNA resulted in covalent binding of these drugs to DNA. The adriamycin semiquinone radical has a greater affinity for DNA and covalent complexes containing up to one adriamycin per 15 nucleotides were obtained. The daunorubicin semiquinone radical, on the other hand, showed a lesser binding affinity and gave rise to complexes in which one drug molecule was covalently bound per 140 nucleotides. Studies with synthetic polynucleotides suggest that these drugs have a high preference for poly (dG) and poly (dC). Microsomal activated drugs also bind covalently to DNA with identical binding affinities. Adriamycin, when injected in rats, also binds covalently to rat liver proteins, RNA and DNA. Microsomal activation of these drugs produced both C7-free radical and C7-quinone methide which act as alkylating agents.